1. Field of the Invention
This invention relates to an aqueous based fluid for use as a completion or workover fluid for oil and gas wells.
2. Description of the Previously Published Art
An important step in the production of oil and gas from wells is the completion of a drilled well. Such completion may comprise numerous subtasks including perforation, cementing, acidizing, fracturing, and others. An important task is the removal of well debris through the use of a completion fluid. The irrigation of a drilled well with a completion fluid washes or floats out various forms of drilling or other debris which could otherwise clog the petroleum producing strata and impede delivery of oil or gas to the production site for recovery. A similar type of fluid is employed during workover of an established well. Workover is a term generic to a process for revitalizing a well to improve its productive performance. Fluid removal of debris following such a workover is usually required; fluids for this purpose share many requirements with completion fluids.
Those skilled in the art will appreciate that numerous requirements may be imposed upon completion and workover fluids depending upon the different conditions found in oil field operations. Ideally, a completion fluid should cause little permeability damage, minimize corrosion, remain stable downhole, weigh up to the desired specific gravity and have the proper viscosity characteristics. It will be understood that conditions down in the bottom of the hole of an oil or gas well are nearly always substantially different from atmospheric temperature and pressure. In general, such locations are under elevated pressure from surrounding strata and experience elevated temperatures. Temperatures in excess of 225.degree.-250.degree. F. are common and much higher temperatures are not uncommon. As wells are sunk to greater and greater depths, higher temperatures and pressures will be experienced.
It has long been appreciated that clear brines can be used for completion and workover. These materials must, in general, have specific gravities or densities higher than that of water. Thus, such fluids commonly comprise aqueous solutions of inorganic salts resulting in densities higher than 8.3 pounds per gallon for water. For example, saturated solutions of potassium chloride, sodium chloride, and calcium chloride have densities of 9.7, 10.0, and 11.6 pounds per gallon respectively. Such elevated densities are needed to maintain a hydrodynamic equilibrium between the downhole pressure and the clear brine completion or workover fluid. Without the aid of this high density there exists the possibility that the well would blow out. Such increased density also aids in the suspension of debris particles for transport out of the well.
The second requirement for these fluids is that of viscosity. It is known that such brines should possess viscosities which facilitate the suspension and transport of debris particles. Accordingly, viscosifiers are generally added to such brines. Examples of these viscosifiers are guar, xanthan, and other gums, modified celluloses, especially hydroxyethyl cellulose (HEC), and biologically modified saccharides such as "XC" polymer (sold by Kelco) and others. One type of preferred material for such viscosifiers are the modified celluloses, especially HEC.
In addition to maintaining viscosity during the completion or workover operations it is frequently desirable, following termination of such procedures, to dissolve or degrade the wall cake. It is known, for example, that acidification of HEC-modified brines will cause hydrolysis of the HEC and the breakdown of the HEC wall cake. It is thus important to maintain the ability to break the HEC-modified brines with acid or otherwise when desired while avoiding degradation in viscosity during use for completion or workover.
A major problem with the employment of fluids made of HEC and brines used at high temperatures, however, has been a relatively rapid breakdown of viscosity of the solutions during use. This breakdown, which is believed to be caused by depolymerization of the cellulosic structure or by other modifications thereof, has diminished the effectiveness of completion and workover applications employing such viscosified brines. The breakdown in viscosity is most dramatically affected by changes in downhole temperature, becoming serious above about 225.degree. F. and critical above about 250.degree. F.
Attempts have been made to stabilize the clear brines. R. H. Rygg in U.S. Pat. No. 4,359,392 discloses the use of copper species such as copper metal, copper salts or organocuprates. However, the resulting brine solution can become expensive and it becomes complicated to formulate and maintain because of the need to add the salts, the viscosifier, and the stabilizer.
In another brine system, T. C. Mondshine in U.S. Pat. No. 4,186,803 uses bridging agents to deposit a thin film of solids to temporarily bridge over the formation pores. His agents are water soluble salts which are insoluble in the saturated brine solution.
In U.S. Pat. No. 4,192,753 D. S. Pye et al disclose an aqueous well completion and workover fluid having many components. They include (1) a particulate resin having a relatively high softening point resin having a softening point of 135.degree. C. or above made of rosin acids, rosin esters, coumarone-indene resins, petroleum resins, polymers derived from one or more terpenes and condensation products of aromatic hydrocarbons with formaldehyde, (2) a water-soluble inorganic salt and (3) a microemulsion. The microemulsion is made of (1) an oil-soluble surface active agent, (2) a water-dispersible surface active agent, (3) wax, and (4) a relatively low softening point resin having a softening point of about 90.degree. to 134.degree. C., where this resin is made of rosin acids, rosin esters, coumarone-indene resins, petroleum resins, polymers derived from one or more terpenes, and condensation products of aromatic hydrocarbons with formaldehyde.